drm/panfrost: move devfreq_init()/fini() in device

Later we will introduce devfreq probing regulator if they
are present. As regulator should be probe only one time we
need to get this logic in the device_init().

panfrost_device is already taking care of devfreq_resume()
and devfreq_suspend(), so it's not totally illogic to move
the devfreq_init() and devfreq_fini() here.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Clément Péron <peron.clem@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200710095409.407087-9-peron.clem@gmail.com